1. Field of the Invention
The present invention relates to an orthogonal frequency division multiplex modem circuit, and in particular, to an OFDM (Orthogonal Frequency Division Multiplex) modem circuit which transmits a plurality of different channels.
2. Description of the Related Art
In recent years, the digitization of broadcasting has been promoted and an OFDM system will be adopted as its modulation system. Moreover, also in a 5-GHz-band wireless LAN (Local Area Network), the OFDM system is adopted as a modulation system.
The OFDM system is a system that divides a transmission signal into pieces, and modulates and transmits a plenty of subcarriers respectively, and has characteristics that the OFDM system has high frequency utilization efficiency and is strong on multi-path fading.
FIG. 10 shows an example of the structure of a conventional orthogonal frequency division multiplex modem circuit. A principle of the above-described OFDM system will be explained with using FIG. 10. First, a transmission signal X is a signal for, for example, digital high-definition television broadcasting, and consists of a 20-Mbps data signal and a 10.72-Mbps overhead (signal for error correction and synchronization control). That is, the transmission signal X is 30.72 Mbps in total.
A 4×512-bit parallel data is generated by passing this signal through a serial/parallel converter (S/P) 101, and the data is divided every 4 bits. Owing to this, a 16-value QAM (Quadrature Amplitude Modulation) baseband signal A is generated.
The 16-value QAM baseband signal A is complex data having a real part (Re) and an imaginary part (Im). Correspondence between each signal point on a complex plane and a 4-bit input signal is shown in FIG. 11.
Owing to this, 512 complex 16-value QAM signals A, each of whose symbol rates is 30.72/4/512 Msps=15 ksps, are outputted. When these 512 complex numbers are inputted into an inverse Fourier transformer (IFFT) 105, 512 sets of transformation results B are obtained. These results B are converted into a serial signal C with a parallel/serial converter (P/S) 106.
With making real parts before transformation be an I signal and making imaginary parts be a Q signal, these signals are outputted to a transmitter (TX) 107 at a sample rate of 15 ksps×512=7.68 Msps. The transmitter 107 performs the orthogonal modulation of the I and Q baseband signals, and outputs them from an antenna 115.
The allocation of the subcarriers in a transmitter signal is shown in FIG. 12. As shown in FIG. 12, each interval between subcarriers is equal to the symbol rate of 15 kHz and the number of subcarriers is 512. Therefore, bandwidth is 15 kHz×512=7.68 MHz.
Next, the structure of a receiving side will be described. In the receiving side, a high frequency signal transmitted from the transmitting side is received with an antenna 116, a receiver (RX) 108 performs an orthogonal demodulation to generate a baseband signal (I, Q) D. A serial parallel converter (S/P) 109 samples this signal at the rate of 7.68 Msps respectively, and generates a parallel signal E consisting of 512 sets of I (real part) and Q (imaginary part) signals. When this signal is inputted into a discrete Fourier transformer (FFT) 110, 512 complex numbers are obtained.
This data F expresses a signal point of each corresponding sub carrier on a complex plane. A corresponding 4-bit data (in the case of a 16-value QAM) is reproduced from this signal point, and is decoded into the original signal Y and outputted with a parallel/serial converter (P/S) 112.
As described above, the bit rate transmitted in the OFDM system is very high-speed, for example, 30.72 Mbps. This is divided into many subcarriers and transmitted. When the number of subcarriers is 512 and a modulation system is the 16-value QAM, a symbol rate per sub carrier becomes only 15 ksps. The duration per one symbol is about 67 μsec, and this is a sufficiently large value (this is equivalent to 20 km) in comparison with the path difference of a usual multi-path. Therefore, the OFDM system has powerful resistance to multi-path transmission.
The OFDM system is now planned with premising the utilization of each single unit such as digital television broadcast and high-speed wireless LAN equipment. However, since the OFDM system has a feature of being essentially strong on the multi-path transmission, this feature is attractive also in other mobile communications.
Therefore, as a natural conclusion, it can be thought that demands for using the OFDM system also for mobile communications come out. However, since the OFDM system realizes vast transmission capacity as a whole by using hundreds of subcarriers, it is not allowed to use this monopolistically by one kind of mobile communication.
Therefore, it is possible to transmit various communications, such as digital TV, wireless LAN, the Internet, and cellular phones, via one OFDM line. The plural kinds of communication signals have different bit rates respectively, and their necessary transmission quality (QoS: Quality of Service) are different according to informational types.
That is, there are various transmission rates (for example, 28.8 kbps, 1.44 Mbps, and 10 Mbps) in data communication, and, an error rate not higher than 10E-6 is required. On the other hand, in speech communication such as a telephone, a transmission rate is 13 kbps or the like, and the error rate of 10E-3 is regarded as sufficient quality.